Satellite radio operators will soon provide digital quality radio broadcast services covering the entire continental United States. These services will offer approximately 100 channels, of which nearly 50 channels in a typical configuration will provide music with the remaining stations offering news, sports, talk and data channels. Digital radio may also be available in limited markets within the United States in the near future from a company that will provide a terrestrial based system using FM subcarrier signals. In Europe, Eureka 147 is the standard for Digital Audio Broadcasting (DAB). It utilizes terrestrial based stations to broadcast digital audio and data to mobile and stationary radios. The European standard expands RF coverage by offering Single Frequency Network (SFN) configurations and implementing Orthogonal Frequency Division Multiplexing (OFDM) modulation to mitigate the vagaries of Raleigh fading.
Satellite radio has the ability to improve terrestrial radio's potential by offering a better audio quality, greater coverage and fewer commercials. Accordingly, in October of 1997, the Federal Communications Commission (FCC) granted two national satellite radio broadcast licenses. The FCC allocated 25 megahertz (MHZ) of the electromagnetic spectrum for satellite digital broadcasting, 12.5 MHz of which are owned by the assignee of the present application “XM Satellite Radio Inc.” and 12.5 MHz of which are owned by another entity.
The system plan for each licensee presently includes transmission of substantially the same program content from two or more geosynchronous or geostationary satellites to both mobile and fixed receivers on the ground. In urban canyons and other high population density areas with limited line-of-sight (LOS) satellite coverage, terrestrial repeaters will broadcast the same program content in order to improve coverage reliability. Some mobile receivers will be capable of simultaneously receiving signals from two satellites and one terrestrial repeater for combined spatial, frequency and time diversity, which provides significant mitigation of multipath interference and addresses reception issues associated with blockage of the satellite signals. In accordance with XM Satellite Radio's unique scheme, the 12.5 MHZ band will be split into 6 slots. Four slots will be used for satellite transmission. The remaining two slots will be used for terrestrial reinforcement.
In accordance with the XM frequency plan, each of two geostationary Hughes 702 satellites will transmit identical or at least similar program content. The signals transmitted with QPSK modulation from each satellite (hereinafter satellite 1 and satellite 2) will be time interleaved to lower the short-term time correlation and to maximize the robustness of the signal. For reliable reception, the LOS signals transmitted from satellite 1 are received, reformatted to Multi-Carrier Modulation (MCM) and rebroadcast by terrestrial repeaters. The assigned 12.5 MHZ bandwidth (hereinafter the “XM” band) is partitioned into two equal ensembles or program groups A and B. The use of two ensembles allows 4096 Mbits/s of total user data to be distributed across the available bandwidth. Each ensemble will be transmitted by each satellite on a separate radio frequency (RF) carrier. Each RF carrier supports up to 50 channels of music or data in Time Division Multiplex (TDM) format. With terrestrial repeaters transmitting an A and a B signal, six total slots are provided, each slot being centered at a different RF carrier frequency.
Referring to FIG. 1, satellite radio operators will soon provide digital radio service to the continental United States. Briefly, the service provided by XM Satellite Radio includes a satellite X-band uplink 11 to two satellites (12 and 14) which provide frequency translation to the S-band for re-transmission to radio receivers (20, 22, 24, and 26) on earth within the coverage area 13. The satellites provide for interleaving and spatial diversity. Radio frequency carriers from one of the satellites are also received by terrestrial repeaters (16 and 18). The content received at the repeaters are also “repeated” at a different S-band carrier to the same radios (20) that are within their respective coverage areas (15 and 17). These terrestrial repeaters facilitate reliable reception in geographic areas where LOS reception from the satellites is obscured by tall buildings, hills, tunnels and other obstructions. The signals transmitted by the satellites 12 and 14 and the repeaters are received by SDARS receivers 20-26. The receivers 20-26 may be located in automobiles, handheld or stationary units for home or office use. The SDARS receivers 20-26 are designed to receive one or both of the satellite signals and the signals from the terrestrial repeaters and combine or select one of the signals as the receiver output.
Current symbol timing recovery for the XM digital audio radio system is based on Amplitude Modulated Synchronization Symbol (AMSS). AMSS can become highly corrupted with high delay spreads and/or when a vehicle is stopping or is stopped. In extreme conditions, no AMSS are detected, leaving the Multi-Carrier Modulation (MCM) demodulator to drift out of lock. In another instance, the channel can have delayed signal(s) that are stronger than the earlier arriving signal. The current AMSS algorithm tends to lock onto the strongest signal rather than the desired earliest signal, causing severe intersymbol interference. Thus, a method and apparatus is needed to provide reliable symbol timing in conditions where current OFDM symbol synchronization is susceptible to corruption or intersymbol interference.